In the field of electrophysiology, there are numerous methods that seek to create blocks of specific electrical pathways for the treatment of atrial arrhythmias. These methods primarily rely on applying energy to the tissue to ablate the tissue and thereby block a pathway for electrical conduction. These techniques are used most often in the left or right atriums and can be used to create electrical block either at discrete sites or along linear paths. Examples of such techniques can be seen in prior art U.S. Pat. Nos. 6,237,605; 6,314,962; 6,527,769, 6,502,576 all of which are incorporated herein by reference.
In a usage that is increasing in popularity, the ablation devices used in these methods are introduced percutaneously and advanced into the right atrium via the vena cava and possibly into the left atrium by a trans-septal sheath. These devices are then maneuvered inside the appropriate chamber of the heart by torquing the shaft of the catheter and deflecting the tip to bring the ablation tip in contact with the desired site. Since the atria are relatively large chambers and are moving with the beating of the heart, it is difficult, however, to position these devices accurately.
The most common method of creating an electrical conduction block is by acutely ablating the tissue at the site where the energy delivery catheter is positioned using RF energy, microwave energy, ultrasonic energy or freezing (i.e., cryoablation). These methods typically involve applying a specified energy to the device resting against the endocardial surface of the tissue for a specified time and then reevaluating the region of the ablation to see if the desired electrical conduction block has been created. The result is the formation of an ablation region that extends from the endocardial surface of the cardiac tissue to a tissue depth that is dependant on the amount of energy that is applied.
These methods have proven to be not only very time consuming but also have become characterized with uncertainty as to whether the appropriate amount of ablation has been performed or if the desired target location is being ablated. For example, the resulting depth of the ablation may extend beyond the targeted tissue and injure adjacent tissue such as the esophagus, the trachea or the bronchial tubes. This can happen because of a varying wall thickness of the target cardiac tissue and because other variables (e.g., variation in the electrical impedance of the tissue) can alter the “burn” depth even when time and energy are controlled.
Other possible adverse outcomes of not successfully ablating the desired target site or ablating too much or too little include: not successfully blocking the electrical pathway, disrupting the wrong pathway or creating stenosis in a vessel such as a pulmonary vein.
Of these adverse outcomes, creating a stenosis is a particularly serious complication. As a result, many doctors try to limit their treatment to the wall of the atrium around the ostium of the pulmonary veins to minimize the risk of creating a stenosis in the pulmonary veins. Stenoses as a result of this type of energy ablation in the pulmonary veins has been reported in a small percentage of cases, however it is uncertain if these stenoses are caused more by excessive ablation, missing the appropriate target site or are an inherent risk for this type of treatment in the pulmonary veins.
Turning back to how the known ablation techniques are implemented, current ablation systems typically are maneuvered by twisting or pushing the device shaft or deflecting the distal end to bring this distal end in contact with the desired site for ablation while this distal end is free in the space of the atrium. The motion of the heart makes it very difficult to accurately control the position of the device in this way. Other systems have been proposed that are intending to seat into the pulmonary veins and then create a circumferential ablation around the pulmonary vein which has been engaged. Examples of such systems are disclosed in U.S. Pat. Nos. 6,117,101 and 6,605,085, each of which are incorporated herein by reference.
These concepts address some of the problems with locating the ablation elements relative to the pulmonary vein but are potentially limited in that they presume a round pulmonary vein and are geared towards making a circumferential ablation around the veins. These systems are not optimally suited for creating focal ablations of discrete points or for making linear lesions other than the circumferential lesions around the device location. They also rely upon these same energy delivery mechanisms as the means to create the electrical block.
There are also systems that use a surgical approach to apply the ablation devices to the endocardial surface of the heart. While these approaches can address some of the limitations discussed above, they require a much more invasive access. They also often are characterized by the same drawbacks discussed above, e.g., determining the correct location, determining the correct depth of burn, etc.
For at least these reasons, there is a need for a system that provides means to create the desired electrical block in the cardiac tissue by ablating the necessary tissue while minimizing the risk of ablating too much or too little of the cardiac tissue. There is also a need for a system that minimizes the risk of ablating structures beyond the targeted cardiac wall.